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The Nice Barrier Reef has misplaced half of its coral prior to now 25 years

A new study found that the warmer ocean temperatures caused by climate change have caused Australia's Great Barrier Reef to lose more than half of its coral since 1995. The researchers say that virtually all coral populations along the Great Barrier Reef have declined due to repeated "bleaching events". over the past 25 years. They said the coral devastation would continue unless action was taken to mitigate the causes of a warming climate.

"We found that the number of small, medium and large corals on the Great Barrier Reef has decreased by more than 50 percent since the 1990s," said co-author Professor Terry Hughes of the ARC Center of Excellence for Coral Reef Studies (CoralCoE). in Queensland, Australia in a press release. "The decline occurred in both shallow and deeper water and in virtually all species."

Hughes added that the decline was most pronounced in branching and tabular corals, the large structural species that provide habitats for fish and other marine life.

The Great Barrier Reef has lost half of its coral in the past three decades. With the loss of more complex coral structures, the habitats for fish are also lost. Photo credit: Andreas Dietzel.

The Great Barrier Reef in the Coral Sea off the coast of Queensland, Australia, is the world's largest reef system and the world's largest single structure of living organisms. It consists of over 2,900 individual reefs and 900 islands that stretch for 2,300 kilometers. Although made up of billions of tiny organisms known as coral polyps, the Great Barrier Reef can be seen from space.

"We used to think the Great Barrier Reef was protected by its size," said Hughes. "However, our results show that even the largest and relatively well-protected reef system in the world is increasingly endangered and in decline."

The Great Barrier Reef as seen from space. Photo credit: NASA

The researchers, led by Dr. Andy Dietzel, also from CoralCoE, examined the health and size of the coral colonies across the reef from 1995 to 2017.

"We measured changes in colony size because population studies are important to understand the demographics and breeding ability of the corals," said Dietzel.

According to NOAA, persistent high temperatures have destroyed parts of the reef from coral bleaching. Coral bleaching occurs when corals are exposed to so much heat that they expel their main food source, the microalgae that live in their cells. This will make the coral look as white as if it was soaked in bleach. Without algae, the reef will also lose its main food source, making it more susceptible to disease and death.

Can coral survive a bleaching event? Unless the stress-induced bleaching is severe, corals have been known to recover. If the algae loss lasts longer and the stress continues, the coral will eventually die. Image credit: NOAA.

Bleaching events on the Great Barrier Reef occurred in 1998, 2001, and 2006, as well as 2008-2009 and 2010-2011, with mass bleaching in 2016 and 2017. Reefs can survive bleaching events if and when the damage is not too great there is enough time between events for recovery. Hughes said it takes about 10 years for a "decent recovery".

However, the continued warming of the ocean can mean that some reefs never return to their previous levels of health, diversity and productivity.

Dietzel said one of the main effects of coral size is its impact on survival and breeding.

"A vibrant coral population has millions of small corals as well as many large ones – the big mums that produce the most larvae," he said. "Our results show that the Great Barrier Reef's ability to recover – its resilience – is compromised compared to the past with fewer babies and fewer large adult breeders."

The team wrote in their paper: “The potential for the restoration of older fertile corals is uncertain given the increasing frequency and intensity of disturbance events. The systematic decline in smaller colonies in regions, habitats, and taxa suggests that a decline in recruitment may have further affected the recreational potential and resilience of coral populations. "

The authors say they will continue to study coral demographic trends because understanding whether the reefs can ever recover is critical to understanding the health of our oceans in a warming world.

"There is no time to lose – we have to reduce greenhouse gas emissions as quickly as possible," the authors concluded in their article.

Further reading:
Article published in the Proceedings of the Royal Society
Press release
Will the Great Brrier Reef recover / NOAA

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